Objective C Calculator Programming Variable - objective-c

Question

Okay, so I've been working on the Stanford iOS development course that they have posted for free online. I've been working on figuring out how to make a programable variable. It has been working fine so far, aside from the fact that I think the following lines of code are programming the variable to be #"x=" instead of the previous number entered.
The View Controller:
#import "ViewController.h"
#import "CalculatorBrain.h"
#interface ViewController()
#property (nonatomic) BOOL userIsInTheMiddleOfEnteringANumber;
#property (nonatomic) BOOL userPressedSomethingElse;
#property (nonatomic, strong) CalculatorBrain *brain;
#end
#implementation ViewController
#synthesize display;
#synthesize inputHistory;
#synthesize userPressedSomethingElse;
#synthesize userIsInTheMiddleOfEnteringANumber;
#synthesize brain = _brain;
- (CalculatorBrain *)brain
{
if (!_brain) _brain = [[CalculatorBrain alloc] init];
return _brain;
}
NSString *xValue = #"0";
- (IBAction)enterPressed
// A specific action if enter is pressed
{
[self.brain pushOperand:[self.display.text doubleValue]];
self.userIsInTheMiddleOfEnteringANumber = NO;
if (self.userPressedSomethingElse)
{
self.inputHistory.text = [self.inputHistory.text stringByAppendingString:#" "];
}
self.userPressedSomethingElse = NO;
}
- (IBAction)variableChanged:(id)sender
{
if (self.userIsInTheMiddleOfEnteringANumber)
{
[self enterPressed];
}
NSString *operation = [sender currentTitle];
xValue = [self.brain programVariable:operation];
self.inputHistory.text = [self.inputHistory.text stringByAppendingString:#"X="];
self.inputHistory.text = [self.inputHistory.text stringByAppendingString:xValue];
}
The Calculator Brain (the .m one):
#import "CalculatorBrain.h"
#interface CalculatorBrain()
#property (nonatomic,strong) NSMutableArray *operandStack;
#end
#implementation CalculatorBrain
#synthesize operandStack = _operandStack;
- (NSMutableArray *) operandStack
{
if (!_operandStack)
{
_operandStack = [[NSMutableArray alloc] init];
}
return _operandStack;
}
- (void) pushOperand:(double)operand
{
NSNumber *operandObject = [NSNumber numberWithDouble:operand];
[self.operandStack addObject:operandObject];
}
- (double)popOperand
{
NSNumber *operandObject = [self.operandStack lastObject];
if (operandObject) [self.operandStack removeLastObject];
return [operandObject doubleValue];
}
- (NSString *) programVariable: (NSString *) operation
{
double result = [self popOperand];
NSString *resultString = [NSString stringWithFormat:#"%.2d",result];
return resultString;
}
The .h Calculator Brain:
#import <Foundation/Foundation.h>
#interface CalculatorBrain : NSObject
- (void) pushOperand: (double) operand;
- (double) performOperation: (NSString *) operation;
- (NSString *) programVariable: (NSString *) operation;
#end
The button that is pushed says "x=", and because of some tracing statements I added, I have figured out that this is being set to xValue. However, I don't know how to fix it... Any ideas?

Answer 1

In your variableChanged:method, you possibly want if (!self.userIsInTheMiddleOfEnteringANumber) at the beginning. Note the logical negation, which I derive the need from the sense of your variable name. I see no way that variable is set, so I trust you are properly setting it elsewhere in your app.
Also, change the variable xValue to a instance variable on ViewController. It is currently a global static variable. If you have more than one ViewController objects created, you will have problems.